Mixtures of organic compounds occur throughout industry, such as in the petrochemical industry, refining industry and elsewhere. Such mixtures include aromatic/aliphatic mixtures, olefin/paraffin mixtures, mixtures containing substituted and unsubstituted hydrocarbons, mixtures containing alcohols, aldehydes, ketones, ethers or esters, mixtures containing fluorocarbons, mixtures containing sulfur-containing compounds, azeotropic mixtures of all kinds, and so on.
Pervaporation is an energy-efficient membrane-separation process that has been used as an alternative to distillation for removal and/or recovery of volatile organic compounds from aqueous solutions and for dehydration of industrial solvents or other organic liquids. The process can provide very selective separation of hydrophobic organic compounds, such as aromatic hydrocarbons or chlorinated solvents, from water, but is much less effective in separating more hydrophilic organics, such as alcohols and ketones, from water, or in separating similar organic compounds from one another.
Refineries and petrochemical plants in the United States use 40,000 distillation columns to separate organic liquid mixtures. These columns account for approximately 3% of total U.S. energy consumption.
In principle, these separations could be performed at a much lower cost and with far less energy consumption by permeation of the liquids or vapors through membranes.
Interest in using pervaporation for separating organic mixtures has waxed and waned over many years. The first systematic studies of pervaporation for separating mixtures of aromatics, or aromatics from aliphatics, were performed by Binning, Lee, Stuckey and others at American Oil in the 1950s. This work is exemplified in U.S. Pat. No. 2,930,754 and other similar patents.
In the 1970's, work on similar separations was carried out by Perry and others at Monsanto. Patents assigned to Monsanto disclose a variety of pervaporation applications. For example, U.S. Pat. No. 3,966,834 concerns separation of dienes from mono-unsaturated compounds.
In the late 1980's and early 1990's, various oil companies—Texaco, Mobil, and particularly Exxon—undertook significant research programs to develop improved membranes and processes for use in aromatic/aliphatic separations. For a few years, Exxon was the most prolific patentee in any membrane-separation-related area on the strength of this effort. Exemplary patents to Schucker and others in this period include U.S. Pat. Nos. 4,929,358 and 5,290,452.
Despite this wealth of research, both in the laboratory and in pilot plants, pervaporation processes that are technically and economically competitive with distillation have not been available to date.
Gas separation by means of membranes has been used for a number of separations involving light organic mixtures, where the organic compounds are gases at ordinary temperatures. Representative organic gas mixtures so treated include natural gas and light petrochemical feedstocks or off gases.
Mixtures of organic compounds in the vapor phase are also found. Vapor separation is a membrane separation process in which a feed stream that is normally liquid under ambient temperature and pressure conditions is supplied to the feed side of the membrane as a vapor. Thus the process is normally performed at elevated temperatures.
A number of patents describe separation of organic gases or vapors by means of membranes. Exemplary patents in this area include U.S. Pat. Nos. 4,553,983; 4,857,078; 5,670,051; 5,769,926 and 6,271,319.